1. Field of the Invention
The present invention relates generally to common mode filtering methods and devices, and more particularly to a method and device for use with different defected ground structures to suppress common mode noises.
2. Description of Related Art
Along with wide application of various kinds of electronic equipments, television networks, switches, mobile communication equipments and office automation equipments, electromagnetic environment of electronic systems is becoming more complicated and EMI (Electromagnetic interference) is becoming an increasingly severe problem, which adversely affects operation of the electronic systems.
Generally, differential signals are used for high-speed data transmission, such as USB 3.0 and IEEE 1394, so as to reduce noise interference. Since differential signals generate much less noise and suppress common mode noise, signal distortion can be avoided. However, coupling noise interference often leads to currents of same phase in two signals, i.e., common mode currents, which are the main source of EMI and should be considered in circuit wiring and design.
EMI is divided into radiated interference and conducted interference according to energy transmission methods. The radiated interference is preferably eliminated by shielding technique and the conducted interference is preferably eliminated and suppressed by magnetic filtering elements. An anti-EMI element can be disposed as close as possible to an interference source so as to efficiently prevent generation of radiated interference.
The most commonly used method for suppressing common mode current is to externally add a common mode choke. The common mode choke comprises two separate coils having a same number of turns and wound on a common magnet, which is equivalent in structure to a magnetic core coil. Such a filter suppresses noise through impedance and frequency characteristics of the magnetic material. The impedance of the magnetic core coil at high frequency is far greater than the impedance at low frequency, and in order to obtain a preferred interference filtering effect, the filter has a maximum impedance at the center frequency of the interference. Combination of self inductance and mutual inductance in a choke leads to a high impedance so as to eliminate common mode noise. However, frequency characteristic and parasitic effect of the ferromagnetic material prevents the common mode choke from operating at frequencies of GHz.
In addition, with the recent progress of multi-layer board fabrication process, a method of controlling the EMI radiation through PCB stack-up is proposed. Although this method has a design concept similar to the common mode choke, its fabrication process is rather complicated and costly.
Therefore, how to provide a common mode filtering method and device with simplified fabrication process, low fabrication cost and good suppression effect has become urgent.